LibJS: Add default constructor for PrivateName

This avoids a round-trip through FlyString("") for every Reference.

Userland/Libraries/LibJS/Runtime/PrivateEnvironment.h

@@ -12,6 +12,7 @@
 namespace JS {
 
 struct PrivateName {
+    PrivateName() = default;
     PrivateName(u64 unique_id, FlyString description)
         : unique_id(unique_id)
         , description(move(description))

Userland/Libraries/LibJS/Runtime/Reference.h

@@ -153,7 +153,7 @@ private:
 
     bool m_is_private { false };
     // FIXME: This can (probably) be an union with m_name.
-    PrivateName m_private_name { 0, "" };
+    PrivateName m_private_name;
 
     Optional<EnvironmentCoordinate> m_environment_coordinate;
 };